In sub-sea surveying, side scan sonars and sub-bottom sonar profilers are towed above the seabottom to produce sonar pictures useful in the identification of seabottom and sub-bottom objects such as pipelines, sunken ships, submarines, mines, torpedos, buoys, and anchoring apparatus. Such sonar systems operate to sonically scan the three-dimensional geography of the bottom and sub-bottom features and produce therefrom a strip chart image resulting from intensity modulating successive marker traces across the paper of a strip chart recorder. The intensity modulation corresponds to received sonar echo intensity and each successive trace represents incremental position of the sonar equipment as it is towed above the bottom. The time between sonar pulse transmission and received echo represents distance to the reflecting object and is mapped into distance on the strip chart by the timed rate of travel of the marking mechanism in successive traces across the chart. Each trace corresponds to a transmitted sonar pulse. There results a photograph like image of the seabottom and its sonar reflecting components.
Such systems are described in the following articles authored by the present inventor, Martin Klein: "New Capabilities for Side Scan Sonar", Oceans '79 IEEE Conference, September, 1979; "Side Scan Sonar", Offshore Services, April, 1979; "Sea Floor Investigations Using Hybrid Analog/Digital Side Scan Sonar", Remote Sensing Conference, Ann Arbor, Mich., April, 1979; "Side Scan Sonar", Undersea Technology, April, 1967.
A cathode ray tube may be substituted for the strip chart recorder to provide much the same graphic representation of seabed features.
While such sonar systems provide a relatively accurate portrayal of the three-dimensional features of the ocean bed, it is oftentimes difficult to identify the nature of articles appearing on the chart. For example, the acoustic energy in a sonar pulse projected towards the ocean bottom may be reflected not only by buried pipelines and other man-made objects, but also by rocks and boulders located beneath the bottom of the ocean. The reflections caused by these naturally occurring geological formations produce traces which approximate those obtained when sonar signals are reflected by the man-made article sought. It would therefore be of advantage to correlate the sonar image with the output of other sensors such as magnetometers which are capable of distinguishing between seabottom rocks and the materials of magnetic susceptibility which often occur in the sought after objects.
Because of the volume of strip chart output in a sonar seabottom search, it would be desirable to have the output of such auxiliary detectors as magnetometers represented directly in graphic form on the strip chart. This, however, is not possible with existing sonar recorder instrumentation because of incompatability of detector output with the format of such recorders which, as noted above, is an intensity modulation of a constant rate of motion marking mechanism. On the other hand the output of the auxiliary sensor is a time varying magnitude which demands that the recorder marking mechanism move under the control of the sensor output magnitude. To provide a trace of the sensor output adjacent to the sonar image would therefore require a separate marking stylus and involve an expensive and inconvenient modification or replacement of the sonar recorders in use today.